The invention is based on a priority application EP 05291595.6 which is hereby incorporated by reference.
The invention relates to an optical wavelength division multiplex transmission system for transmitting optical signals on a plurality of wavelength channels, having a transmitting terminal and a receiving terminal interconnected by a transmission line with at least one, preferably a plurality of optical amplifiers, each of the optical amplifiers comprising a Raman amplifier with at least one pump light source for pumping the transmission line upstream the optical amplifier.
Current optical transmission systems, in particular submarine systems, are generally based on optical amplifiers in the form of Erbium Doped Fiber Amplifiers (EDFAs) and use a low frequency modulation of the signal power (frequency around 150 KHz) on the transmitting terminal side to check the status of each optical amplifier in the system. The optical amplifiers answer with a low frequency modulation (<100 KHz) of their pump which can be interpreted by the receiving terminal. The amount of information to be exchanged in this way is very low in an EDFA based transmission system. The desired system gain flatness is obtained owing to the design of the amplifier (fiber length, span loss, gain flattening filter). When the system loss increases due to ageing or fiber repairs, a tilt of the system gain profile appears, which can be compensated by means of a link filter.
In the case of long transmission spans between individual EDFAs, these can be supported by Raman amplifiers perfoming distributed Raman amplification on the transmission spans in question. Prior art optical transmission systems of this type are known from the documents U.S. Pat. No. 6,532,102 B2, U.S. Pat. No. 6,721,091 B2, and WO 2004/073215 A1, respectively, each of which discloses an optical transmission system with hybrid Raman/EDFA amplifiers.
In order to increase the flexibility of the amount of data that can be exchanged between the transmitting terminal and the optical amplifiers, solutions based on the use of an optical supervisory channel (OSC) have been realised. Examples of such solutions are disclosed in the above-mentioned documents U.S. Pat. No. 6,532,102 B2 and WO 2004/073215 A1.
However, a common drawback of the prior art EDFA based or hybrid transmission systems is the inevitable use of a plurality of tap couplers for extracting and adding data from/to the optical supervisory channel and for controlling operating parameters of the optical amplifiers, e.g. pump powers of the Raman amplifiers. This entails the use of a plurality of optical multiplexers and de-multiplexers in each optical amplifier along the transmission line, which deteriorate the system performance by introducing additional loss and noise together with increased Polarisation Mode Dispersion (PMD) and Polarisation Dependent Loss (PDL).